One thing no one has pointed out is that a beater car usually holds it's value pretty well, what's the differance between a car with 110k miles and one with 150k miles. May only be a few thousand. Compare that to the depreciation/loss of owning a super charger.
That being said, I personally think when a car like the M3 exists, why own two (unless you have utility needs like plowing/off roading/hauling). If you want something sportier, but don't want to sacrifice daily driving much, you can lightly mod. If you want something more comfortable, get a 335i w/o sport suspension, or a 535. No need to own two cars IMHO.